In recent years, along with the popularization of information equipment, there has been a growing demand for higher levels of performance of display panels (liquid crystal display devices). In a liquid crystal display device, the passage and blockage of transmitted light is controlled by changing the direction of alignment of the liquid crystal molecules according to applied voltage. For the liquid crystal display device to have a higher level of performance, the state of alignment of the liquid crystal molecules in the absence of voltage being applied to the liquid crystals is important. Therefore, for a satisfactory state of alignment, it is necessary to control an initial formed angle (pretilt angle) between the liquid crystal layer and the liquid crystal molecules.
A known example of a method for controlling a pretilt angle is a technique disclosed in Patent Literature 1 listed below, i.e., a technique so called PSA. PSA is a technique by which the direction in which the liquid crystals lean is memorized by mixing polymerizable monomers into the liquid crystals and polymerizing the monomers with light, heat, or the like in the presence of voltage being applied to the liquid crystals. For example, first, a liquid crystal material containing monomers is injected into a cell obtained by so joining two substrates each provided with an alignment film to each other that the alignment films face each other. Then, the monomers are polymerized by irradiating them with ultraviolet rays with the liquid crystal molecules aligned along a predetermined direction, for example, by applying an electric field to the cell.
Thus formed on each of the alignment films is a polymer layer having a tilt. As a result, those liquid crystal molecules in contact with the polymer layer can be fixed with a pretilt angle given thereto. According to PSA, even a pixel structure that is weak in alignment controllability for the purpose of a larger aperture ratio or the like becomes faster in response speed and unlikely to suffer from a disturbance in liquid crystal alignment, for example, even when pressed with a finger.
Furthermore, at the step of aligning liquid crystal molecules according to PSA (such a step being hereinafter called “PSA step”), a method for applying voltage is important because when there is a variation in magnitude of voltage that is applied during ultraviolet irradiation, there occurs a difference in pretilt angle that leads to a variation in transmittance characteristic. For this reason, in Patent Literature 2 listed below, for example, there is proposed a method for manufacturing a liquid crystal display device through a PSA step under a scheme to avoid the influence of a defect in wiring by driving the liquid crystals through the use of a capacitor formed by AC application, i.e., under a Cs-COM voltage application scheme.
According to the method for manufacturing a liquid crystal display device described in Patent Literature 2, a first substrate is provided with a common electrode through which voltage is applied to the entire substrate, and a second substrate has gate bus lines and data bus lines disposed thereon in a matrix manner. Provided at an intersection between two bus lines are a thin-film transistor, a pixel electrode connected thereto, and a Cs bus line that forms a capacitor with the pixel electrode. Further provided in a space between the first and second substrates is a liquid crystal layer formed by filling the space with a liquid crystal composition containing a photosensitive material, with a capacitor formed by the common electrode and the pixel electrode with the liquid crystal layer sandwiched therebetween. Moreover, according to the method for manufacturing a liquid crystal display device described in Patent Literature 2, the liquid crystal layer is irradiated with light by applying AC voltage between the common electrode and the pixel electrode.
Thus, during application of voltage to the liquid crystals, the voltage is written by applying it between two common electrodes, instead of being written through a data bus line. This makes it possible to prevent a problem associated with a defective part made by a breakage or short circuit in a bus line during writing through the data bus line, i.e., to prevent such an obstacle that only the defective part shows a different level of brightness due to a pretilt angle formed in the defective part.